Polymers are commonly extruded or injection molded to make films, sheets, preforms, and other molded articles useful for a wide range of applications including containers such as bottles and cups; wrappers for packaging; various displays; signs; and multiple other products. Often additional layers are coextruded with a base layer to improve certain properties. Other times additional layers are coated onto a base layer. Such additional layers can be made from a different material than the base layer to improve particular properties such as barrier properties which limit migration of various liquids or gases. Alternatively, the additional layers could be made substantially of the same resin as the base layer, but contain an additive designed to improve certain properties, such as the addition of a UV absorber to minimize degradation of the base material. Control of the layer thickness is often critical in terms of performance and cost. A major problem in the industry is the measurement of the thickness of polymer layers on line during the extrusion, coating, injection molding, or other manufacturing process.
The need for thickness control is particularly important for multi-layer preforms used to make containers, such as bottles for beer, soda, or other carbonated beverages. The preform is typically blow molded to make the container. Such multi-layer plastic containers often contain a thin layer which provides a barrier against migration of gases such as carbon dioxide or oxygen. Because the layer is thin, it is critical that the layer be uniform in thickness; a non-uniformity such as a thin section or a pinhole could substantially degrade the barrier properties—significantly decreasing the performance of the resultant container.
Measurement of the thickness of polymer layers on line has been done using optical transmission techniques, whereby light from a UV, visible, or IR source is passed through the sample, and a component of the layer being measured selectively absorbs some of the light. U.S. Pat. No. 4,691,231 discloses a method and an apparatus for inspecting the sidewalls of containers such as returnable soft drink or beer bottles. The bottles are illuminated with front lighting and back lighting. Reflection and transmission of the light is recorded by six different video scan fields. The signals are examined to determine the presence of dirt, other foreign material, scuffs or other defects on the sidewalls of the bottles.
This technique is not useful when the structure contains opaque material, and the incident beam of light is blocked from passing through the sample across the wavelengths of interest.
Measurement of layers on-line has also been done using interference techniques, whereby light is passed through the sample, and the light is reflected from the various interfaces. The reflections can constructively or destructively interfere depending on the index of refraction of the layers, and the thickness of the layers. This technique is extremely difficult in practice because the difference in index of refraction at the interface between the layers must be great enough to reflect light, the interface must be very sharp (no interdiffusion between the layers) and the thickness of the layers must be very uniform over the sample area measured. Such conditions are difficult to achieve.
Measurements of wall thickness of a container has been performed using optical transmission or reflection techniques wherein a container wall is illuminated with front and back light. Reflected light and transmitted light are imaged and converted to a measurement of wall thickness.
Other techniques for measuring wall thickness of a container include using the dielectric properties of the container wall in conjunction with a capacitance sensor. U.S. Pat. Nos. 5,155,443 and 5,558,233 provide methods and apparatuses for determining the wall thickness of containers composed of a dielectric material.
In addition to the above techniques for measuring thickness, other information about plastic layers and containers has been obtained by adding fluorescent dyes to precursor polymer compositions. U.S. Pat. No. 5,201,921 provides a process for rendering a plastic identifiable by adding 5 to 10,000 ppb of a fluorescent marker to the plastic during or after its manufacture. The patent describes the use of the process for marking, identifying, and sorting plastic bottles. The process can be used for sorting plastic bottles in preparation for recycling.
U.S. Pat. Nos. 5,292,855; 5,336,714; & 5,423,432 provide water-dissipatable, sulfo-containing polyesters and polyesteramides having copolymerized therein thermally stable near infrared fluorophoric compounds. The polymers can be used to create a coating or ink which may be used to mark articles for the purpose of detection or identification.
U.S. Pat. Nos. 5,397,819; 5,461,136; 5,553,714; & 5,703,229 provide novel near infrared (NIR) fluorophores. The fluorophores are phthalocyanine and naphthalocyanine derivatives which are covalently bonded to substituted silicon and aluminum compounds which fluoresce in the near infrared range. The patents also provide a method using the fluorophores to mark thermoplastic compositions and containers made from such compositions. The NIR fluorophores can be excited and the ensuing fluorescence can be detected. The detected fluorescence is used to identify and sort polymers and containers. The process is particularly useful for the recycling used containers.
U.S. Pat. No. 5,614,008 provides inks which comprise a water-dissipatable polyester, NIR fluorophores, a humectant, a lower aliphatic alcohol, and water. The inks, which are nearly invisible to the human eye, can be used to mark the surface of articles for identification, authentication, sorting, etc.
U.S. Pat. No. 5,783,307 provides a UV stabilized multi-layer structure which has an easily visible UV protective layer and an underlying polymeric layer. The UV protective layer contains a UV absorber which may be a fluorescent material. It also contains an optical brightener which has fluorescent properties to make the UV protective layer visibly illuminated upon exposure to a UV or a white light source.
U.S. Pat. No. 5,804,447 provides the use of certain compounds as markers for liquids. The compounds are from the class of the phthalocyanines, naphthalocyanines, nickel dithiolene complexes, aminium compounds of aromatic amines, methine dyes or azule-nesquaric acid dyes. The compounds have absorption maximum in the range of 600 to 1200 nm and/or fluorescence maximum in the range of 620 to 1200 nm. The absorption and/or fluorescence wavelengths are detected and used to identify previously marked liquids.
U.S. Pat. No. 4,305,816 provides a method and apparatus for testing open ended containers. The patent mentions aluminum containers and the testing thereof for wall integrity. The patent provides lighting of the container walls using a quartz source. Defects are detected by photodiodes which detect light transmitted through the wall of the container.